The present invention relates to a process for producing a cryogenic aerosol and controlling the physical properties of a cryogenic aerosol.
The present invention has been developed for its applicability to the semi-conductor and micro-electronics industries and in particular to the cleaning of contaminated substrates, including, for example, semiconductor wafers of silicon and of gallium arsenide, multiple chip carriers, flat panel displays, magnetic hard disks, MEMs (microelectromechanical systems) and other electronic devices. Many methods have been developed to clean such surfaces. Techniques include the use of solvents or chemical cleaning for removing contaminant films from surfaces, the use of high energy sonic waves, and combinations thereof. Solvents for chemicals may be applied as gas jets or liquid spray. Aerosol sprays using frozen water vapor have been disclosed by Tada et al. (U.S. Pat. No. 5,035,750, U.S. Pat. No. 5,025,597 and U.S. Pat. No. 4,974,375).
More recently, cryogenic aerosols have been developed for jet spraying against surfaces, particularly within the semi-conductor industry to facilitate particulate decontamination as described in McDermott et al. (U.S. Pat. No. 5,294,261) and Tamai et al. (U.S. Pat. No. 5,512,106), both of whose contents are incorporated herein by reference, and for the elimination of stiction as described in copending application assigned to the same assignee hereof filed concurrently herewith, entitled "Eliminating Stiction with the Use of Cryogenic Aerosol", application Ser. No. 08/823,270 incorporated herein by reference.
Cryogenic aerosols are formed by delivering a gaseous and/or liquid cryogen to a nozzle. Upon expelling the mixture from the nozzle, the cryogenic aerosol is formed as described in copending U.S. application Ser. No. 08/773,489, filed Dec. 23, 1996 and incorporated herein by reference. The cryogenic aerosol contains at least substantially solid aerosol particles of the cryogen. The term "aerosol particles" as used herein refers to droplets comprised of liquid and/or solid generally of about 0.01 to about 100 microns in diameter or larger. The aerosol particles may further be partially solid or partially liquid.
Typically, cryogenic aerosols are formed from chemicals such as argon, nitrogen, carbon dioxide and mixtures thereof. Argon and nitrogen, both of which are inert, are the preferred components of cryogenic aerosols. Other inert chemicals may be used as well.
Until now, cryogenic aerosols have been formed by expanding the gaseous and/or liquid cryogen into a process chamber at a pressure upwards of 2.times.10.sup.4 Pascal. At such pressures, cryogenic aerosols are characterized by high mass and low velocity aerosol particles. Also, the aerosol density (number of aerosol particles per unit volume) is low. Treatment of surfaces with these high mass aerosol particles can result in insufficient cleaning, especially for contaminant particles five microns in diameter or smaller. As the semiconductor industry moves to ever smaller geometries, the need for `cleaner` substrates becomes more pronounced. While present standards allow for up to 1700 contaminant particles of size 0.12 microns and greater per square meter, it is projected that by 2010, industry standards will allow for no more than 150 contaminant particles of size 0.02 microns and greater per square meter. With these increasingly stringent industry demands, there is a need for more efficient treatment methods, in particular, treatment methods that are capable of successfully treating surfaces with small contaminant particles.